This study is designed to determine whether or not humans can be conditioned to a stimulus that will result in activation of their immune system. The unconditioned stimulus is interferon gamma, which has known effects on the immune system that can be measured in serum. The conditioned stimulus is a distinctive-tasting substance, propylene glycol. The unconditioned and conditioned stimuli are paired in a particular schedule to test whether the immune system can be conditioned and the propylene glycol can, by itself, elicit an immune response. To date, 11 volunteers have completed the four-week study. All the blood samples will be analyzed together at the completion of the study. Therefore, there are no results at this time.